1. Field
Example embodiments relate to a photomask used in a photolithography process in the fabrication of semiconductors. Other example embodiments relate to a reflective photomask.
2. Description of the Related Art
With the development of nanotechnologies, the degree of integration of semiconductor devices is increasing. The result of the increase in the degree of integration of semiconductor devices is that sizes of transistors or other unit elements and wires in the semiconductor devices become further decreased. The next-generation semiconductor technologies may be improved using process techniques for efficiently forming fine patterns, rather than design techniques. The technical ability to form fine patterns may vary based on a photolithography process which is basic in forming a pattern. In the photolithography process including forming a photoresist film on a wafer, transferring information about a pattern to be formed using light, and patterning the photoresist film using a developing solution, light may be the important variable.
The resolution at which it is possible to form a fine pattern depends on the wavelength of light used. At present, the photolithography process may have used i-line at about 365 nm, KrF laser at about 248 nm, and ArF laser at about 193 nm. According to the development of photolithography techniques, the resolution, conventionally limited by the wavelength of light used, may have greatly improved to the extent of forming a pattern corresponding to about ½ of the wavelength. Semiconductor device integration technologies have been developed such that the pattern may not be formed more finely using conventional light sources (ArF, KrF and/or i-line), thus there is a need for a novel light source for use in a semiconductor device fabrication process. Thorough research into the application of EUV (Extreme Ultra-Violet) light, referred to as soft X-rays, to semiconductor device fabrication processes may have been conducted.
EUV light may have a wavelength of about 13.5 nm, which is smaller than the wavelengths of conventional light sources (ArF: about 193 nm, KrF: about 248 nm and i-line: about 365 nm). Because the wavelength of light is directly linked with the resolution of a pattern, the use of EUV light is receiving attention because greater resolution is possible than when using conventional light sources. EUV light is very sensitive and may have increased energy, due to the decreased wavelength thereof, and thus, may not be used for a conventional transparent mask. The transparent substrate of the conventional transparent mask must be subject to increased energy, and the energy efficiency of light may be decreased, leading to decreased resolution. A reflective photomask may have been developed. Such a reflective photomask, which reflects light without transmission, may be subject to less energy due to the reflection of light.
The photomask for EUV light may not be a transparent type but a reflective type, unlike conventional photomasks, and may be considered to be a mirror similar to a photomask reflecting incident light. Such a photomask may be an extension of conventional photomask techniques and may be referred to as a photomask, instead of a photomirror and/or an optical mirror. A conventional reflective photomask is described below.
FIG. 1 is a longitudinal sectional view schematically showing the conventional reflective photomask 100. As shown in FIG. 1, the conventional reflective mask 100 may include a reflective layer 120 formed on a substrate 110, a capping layer 130 formed on the reflective layer 120, a buffer layer 140 formed on the capping layer 130, and an absorbing layer 150 formed on the buffer layer 140. The reflective layer 120 may function to reflect incident EUV light, and the capping layer 130 may act to protect the reflective layer 120 from external physical and chemical damage. The buffer layer 140 may be used to increase adhesion between the capping layer 130 and the absorbing layer 150, and the absorbing layer 150 may function to absorb incident EUV light so as not to reflect the EUV light.
The conventional reflective photomask 100 of FIG. 1 may be completed by sequentially forming the reflective layer 120, the capping layer 130, the buffer layer 140, and the absorbing layer 150, on the substrate 110, and then patterning the absorbing layer 150 and the buffer layer 140. During the fabrication process, the capping layer 130 and the reflective layer 120 may be frequently damaged. The absorbing layer 150 may be formed of chromium and/or tantalum nitride. The buffer layer 140 may be formed of ruthenium and/or chromium nitride. The capping layer 130 may be formed of silicon and/or ruthenium, and the reflective layer 120 may be formed into a multilayered structure including pluralities of silicon layers and molybdenum layers.
In the conventional reflective photomask 100, because the pattern of the absorbing layer 150 and the buffer layer 140 is particularly fine, more stable process conditions may be required upon the formation of the pattern. The selectivity of the photoresist and the absorbing layer 150 upon patterning of the absorbing layer 150, the selectivity of the absorbing layer 150 and the buffer layer 140 upon patterning of the buffer layer 140, and the selectivity of the buffer layer 140 and the capping layer 130 may be difficult to ensure. Where defects (e.g., dark lines and/or pinholes) are created in the absorbing layer 150 and/or the buffer layer 140, the defects may be corrected. The correction process may be used to etch the dark lines using laser and/or ion beams or to cover the pinholes with gallium and/or other materials. During the correction process, the capping layer 130 and the reflective layer 120 may not become more damaged. Because the capping layer 130 and the reflective layer 120 may be further damaged by a chemical cleaning solution upon a cleaning process involved in the fabrication process or correction process, the prevention or reduction of damage to these layers during the cleaning process may be required.
Where the capping layer 130 or the reflective layer 120 is further damaged, light, which is reflected from the damaged portion, may not be reflected through a desired path, such that the pattern may not be uniformly transferred to the wafer. Accordingly, various other materials have been used for the absorbing layer 150, the buffer layer 140, and the capping layer 130 in order to provide more stable etching selectivity and exhibit improved resistance to the correction process and cleaning process. The materials in terms of adhesion of the layers, similar coefficients of thermal expansion, relatively easy patterning and correction, and the price of materials are not easy to replace. Further, although extensive effort to convert the fabrication process into other methods has been conducted, such methods may not use conventional processes and equipment and require the introduction of new equipment, and thus may be difficult to realize.